1. Field of the Invention
This invention relates generally to a printed antenna, and, more particularly, to a planar printed slot antenna that includes two or more curved antenna elements interlaced to reduce the overall size of the antenna, where the spacing, feed locations, and length of the elements are selected to electromagnetically couple the elements to increase the antenna""s bandwidth and achieve an omni-directional radiation pattern.
2. Discussion of the Related Art
There is a growing demand for wireless communications services, such as cellular telephone, personal communications systems (PCS), global positioning systems (GPS), etc. With this demand there is a need for low-cost miniaturized planar antennas. The multitude of wireless services requires multiple antennas to cover different frequency bands and functions. Also, the demand for dualband phones is ever growing as people increasingly tend to use both analog and digital communications services. Further, both cellular phone and PCS antennas require an omni-directional pattern.
Additionally, it is desirable that the size of the communication apparatus and the transmitting or receiving antennas be small. This becomes even more of a necessity when multiple antennas have to be mounted in a limited area. In military applications, a small antenna size is critical for low radar visibility, and to increase system survivability. In commercial applications, small size alleviates problems with styling, vandalism and aerodynamic performance. Size reduction is especially useful in low frequency applications in the HF, VHF, UHF and L frequency bands ranging from 30 to 3000 MHz. The wavelengths in these bands range from 1 km to 10 cm. Considering the fact that a resonant dipole is about a half-wavelength long, the motivation behind size reduction is obvious.
For low frequency applications, low-profile printed antennas include printed microstrip dipole and printed slot antennas. Printed antennas essentially comprise a printed circuit board with a trace layout. The trace layouts can be made using chemical etching, milling or other known methods. These antennas enjoy a host of advantages including ease of manufacture, low cost, low profile, conformality, etc.
U.S. Pat. No. 6,081,239 issued Jun. 27, 2000 to Sabet et al. discloses a planar printed antenna that employs a high dielectric superstrate lens having a plurality of air voids that set the effective dielectric constant of the material of the lens to reduce resonant waves in the lens, thus reducing power loss in the antenna. The superstrate with air voids allows the size of the dipoles or slots to be reduced for any particular frequency band.
FIGS. 1(a) and 1(b) show a known slot antenna 10 including a metallized ground plane 16 and microstrip feed line 12 printed on opposite sides of a printed circuit board (PCB) 14. A linear slot element 18 is cut out of the ground plane 16 by a suitable etching step or the like. The microstrip line 12 is connected to the ground plane 16 at the edge of the slot element 18 by a shorting pin 20 extending through the circuit board 14.
It is possible to reduce the area occupied by a linear antenna element 22 by bending or winding the antenna element 22 into a curved or twisted shape, as shown in FIGS. 2(a) and 2(b). However, bending the antenna element 22 immediately results in a sharp reduction of its bandwidth. This can be verified by numerical modeling and computer simulation.
FIG. 3 shows the effect of gradually bending a slot antenna element 24 and how it affects the antenna bandwidth, near field, and vertical and horizontal polarization. This simulation shows that more windings result in a more omni-directional antenna pattern, but the bandwidth of the antenna element 24 is reduced.
A wound slot antenna element has to be fed at a location close to its end because the input impedance at its center is very high. The antenna element can be fed using a microstrip line printed on the other side of the substrate with a matching extension or a shorted via hole, as shown in FIGS. 1(a) and 1(b). A coaxial cable can also be used, where its outer conductor is connected to the ground area of the slot antenna and its inner conductor is shorted through the slot.
As discussed above, an antenna design challenge is to increase or maintain the bandwidth of a printed antenna while at the same time reducing the size of the antenna by winding the antenna elements. It is therefore an object of the present invention to provide an omni-directional printed antenna that has these advantages.
In accordance with the teachings of the present invention, an omni-directional printed antenna is disclosed that includes at least two wound slot antenna elements on a small ground plane. The spacing between the elements, the lengths of the elements and the feed location of the elements are selected to provide a desirable electromagnetic coupling between the elements that causes the narrow bandwidth of the individual elements to combine into a wide bandwidth, while retaining an omni-directional radiation pattern. Winding the elements together in this manner also allows separate antennas for different frequency bands to be combined as a single multi-band antenna in a small location. Further, the printed antenna can be patterned on a copper tape or foil to create a sticker type antenna that can be readily mounted on non-planar surfaces. The antenna can also be deposited as a conductive coating on a high permittivity ceramic to further reduce the antenna size.
Additional objects, advantages, and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.